1. Field of the Invention
The present invention relates to secondary ion mass spectrometry using slow, highly charged ions.
2. Description of Related Art
The requirements on surface analytical techniques are becoming more stringent, particularly as the feature size of semiconductor devices continues to decrease. Among the currently available techniques, secondary ion mass spectroscopy (SIMS) is highly favored because it offers in-depth information, low detection limits, and high depth resolution. In standard SIMS, a primary beam of energetic singly charged ions strikes a sample surface, which releases electrons and secondary ions. Typically, the sputter yield is about 2-10 sample atoms per incident ion, and the secondary ion yield per incident ion is often less than 10.sup.-2. The number of secondary ion counts per unit of sample consumption primarily determines the sensitivity limit of SIMS. In the case of surface analysis by static SIMS, values for sensitivity limits are on the order of 10.sup.9 atoms/cm.sup.2.
The atomic and molecular secondary ions emanating from the sample are introduced into a mass analyzer; both positive and negative ion mass spectra of the species present in the surface can be measured. While the molecular ions can dissociate, these fragment ions are usually not distinguished and give rise to some background. The SIMS spectrum contains secondary ions that are stable to dissociation and the ionic fragments of those that are not. The composition of a microscopic region on the surface of the solid sample can thus be elucidated. Instruments for conducting SIMS are broadly classified into two types: a scanning type that scans an analyzed region with a sharply focused primary beam to obtain an ion image, and a direct imaging type that bombards the whole analyzed region with a primary beam of a relatively large diameter and obtains an ion image on the principle of an ion microscope.
Limitations in standard SIMS are becoming apparent in more advanced applications. Development of the next generations of semiconductor devices will require much improved characterization techniques. Thus, a need exists to develop SIMS with at least an order of magnitude greater sensitivity. The present invention addresses the limitations of conventional SIMS by using enhanced sputtering by slow, highly charged ions.